The set-up of a laser communications link involves the proper alignment of numerous optical components. One of the more important and essential tasks which needs to be accomplished in the set-up of a laser communications link is the proper focus and alignment of the in-coming laser beam at the receiver. Specifically, it is essential for the laser beam to be properly aligned and focused onto the detector of the laser beam receiver. For several reasons this is not an easy task.
Typically, the detector of a laser beam receiver includes an avalanche photo diode (APD) which is capable of amplifying the incoming light so that the informational data which is being carried thereon can be easily retrieved and used. Of no small concern is the ability of the detector to handle the data that is carried on the laser beam. It happens that detectors with active areas (also referred to as apertures) which have diameters of approximately five hundred microns (500 .mu.m) are capable of handling about four to five hundred Megabits per second (400-500 Mb/s). It is known, however, that smaller apertures are capable of handling even larger volumes of data. Specifically, it is known that detectors with active areas which are approximately one hundred and fifty microns in diameter (150 .mu.m) are capable of handling laser beam data transmission that are somewhere between six hundred and twenty two Megabits per second and one and two tenths Gigabits per second (622 Mb/s-1.2 Gb/s). This increase is significant and cannot be ignored. With the use of a smaller aperture (active area), however, there is increased difficulty in establishing the focus of the laser beam on the aperture.
The nature of the problem involved in establishing and maintaining the focus of a laser beam on the aperture of a detector in a laser beam receiver is at least three-fold. First, the use of a very small aperture (i.e. 150 .mu.m diameter) requires a very precise alignment of the laser beam focal point with the aperture. Second, due to the small scales that are involved, alignment adjustments of the laser beam's focal point is a sensitive operation which requires effective monitoring. Third, without specifically dedicated equipment, the compactness and confined space requirements of a laser beam receiver make direct visual references for the alignment of the laser beam focal point with the detector aperture virtually impossible.
In light of the above it is an object of the present invention to provide a laser beam receiver, and a method for aligning the optical components of such a receiver, which use light reflected out of the laser beam path to reference the detector aperture, and to establish the location of the laser beam focal point relative to the aperture. It is another object of the present invention to provide a laser beam receiver, and a method for aligning the optical components of such a receiver, which establish a common grid coordinate system for locating both the detector aperture and the focal point of the laser beam on the detector relative to the aperture. Still another object of the present invention is to provide a laser beam receiver, and a method for aligning the optical components of such a receiver, which allow for visually controlling the remote adjustment of optical components without removing or disrupting other components. Another object of the present invention is to provide a laser beam receiver, and a method for aligning the optical components of such a receiver, which use light collected from behind an obscuration in the beam to reference the detector aperture and to establish the laser beam focal point relative to the aperture. Yet another object of the present invention is to provide a laser beam receiver with properly aligned optical components which is simple to use, relatively easy to manufacture, and comparatively cost effective.